When you learned how to translate simple English statements into mathematical expressions, you learned that "of" can indicate
"times". This frequently comes up when using percentages.

If you need to find 16% of 1400,
you first convert the percentage "16%" to its decimal form; namely, the number "0.16".
(When you are doing actual math, you need to use actual numbers. Always convert the percentages
to decimals!) Then, since "sixteen percent OF fourteen hundred" tells you to multiply
the 0.16 and the 1400, you get: (0.16)(1400)
= 224. This says that 224 is sixteen percent
of 1400.

Percentage problems usually work off of some version
of the sentence "(this) is (some percentage) of (that)", which translates to "(this)
= (some decimal) × (that)". You will be given two of the values, or at least enough information
that you can figure two of them out. Then you'll need to pick a variable for the value you don't
have, write an equation, and solve for that variable.

What percent of 20 is 30?

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We have the original number (20) and the comparative
number (30). The unknown in this problem is the rate or percentage. Since the statement
is "(thirty) is (some percentage) of (twenty)", then the variable stands for the percentage,
and the equation is:

30 = (x)(20)

30 ÷ 20 = x = 1.5

Since x stands for a percentage,
I need to remember to convert this decimal back into a percentage:

1.5 = 150%

Thirty is 150% of 20.

What is 35% of 80?

Here we have the rate (35%) and the original
number (80); the unknown is the comparative number which constitutes 35% of 80. Since the exercise statement is "(some number) is (thirty-five percent)
of (eighty)", then the variable stands for a number and the equation is:

x = (0.35)(80)

x = 28

Twenty-eight is 35% of 80.

45% of what is 9?

Here we have the rate (45%) and the comparative
number (9); the unknown is the original number that 9 is 45% of. The statement is "(nine) is (forty-five percent) of (some number)", so the variable
stands for a number, and the equation is:

9 = (0.45)(x)

9 ÷ 0.45 = x = 20

Nine is 45% of 20.

The format displayed above, "(this number)
is (some percent) of (that number)", always holds true for percents. In any given problem,
you plug your known values into this equation, and then you solve for whatever is left.

Suppose you bought something that was priced
at $6.95, and the total bill was $7.61. What is the sales tax
rate in this city? (Round answer to one decimal place.)

The sales tax is a certain percentage of the price,
so I first have to figure what the actual tax was. The tax was:

7.61 – 6.95 = 0.66

Then (the sales tax) is (some percentage) of (the
price), or, in mathematical terms:

0.66 = (x)(6.95)

Solving for x, I get:

0.66 ÷ 6.95 = x = 0.094964028...
= 9.4964028...%

The sales tax rate is 9.5%.

In the above example, I first had to figure out
what the actual tax was. Many percentage problems are really "two-part-ers" like this:
they involve some kind of increase or decrease relative to some original value. Warning: Always
figure the percentage of change relative to the original value.

Suppose a certain item used to sell for seventy-five
cents a pound, you see that it's been marked up to eighty-one cents a pound. What is the percent
increase?